1) Field of the Invention
This invention relates to pneumatic tires and more specifically to tubeless tires that more effectively control inflation air loss rate during nominal operating conditions and impede the aging of materials.
2) Description of Related Art
The ability of a pneumatic tire to maintain its inflation pressure has a major effect on the performance and endurance of the tire. The tire designer selects the position, strength and configuration of various components within the tire based on an assumed tire shape which is controlled during use of the tire.
For example, a tire which is not properly inflated can have significant overall wear or significant irregular wear over the tread width. It is also well known in the art that increasing the inflation pressure lowers the rolling resistance to achieve less fuel consumption.
The vehicle operator is expected to maintain the proper inflation pressure in each tire. Frequent inspections of the tires and a checking of inflation pressure is required. It is known that air permeates from within a tire at a slow rate. However, over time this can result in a significant loss in inflation pressure. For example, a truck tire starting with an 85 psi inflation pressure can have a 2-3 psi inflation pressure loss per month, due to air permeability through the tire. U.S. Pat. No. 4,938,056 discloses maximum air loss requirements for passenger car tires. For example, a P205/75R15 tire should have a maximum loss rate of no more than 2.5 percent per month.
Tires of the well known tubeless variety maintain their inflation pressure by using a thin innerliner rubber ply which is bonded to the inside of the tire and extends from one bead segment to the axially opposite bead segment. This innerliner ply is made from a rubber composition having an excellent air retention capability. That is, the "permeability" of the innerliner ply is very low. It is not unusual for this innerliner ply material to have a permeability one order of magnitude less than that of the other rubber components of the tire. To avoid adding additional mass to the tire, the most efficient innerliner plies are those that have the lowest permeability at a minimum thickness controllable in the manufacturing process.
The most commonly used rubber compounds for the tire's innerliner ply are of a butyl or halobutyl composition having a sulfur or zinc oxide cure system. Typically, the halobutyl is a bromobutyl or chlorobutyl rubber. Other innerliner plies are composed of blends of rubber to achieve a thinner innerliner ply, such as those disclosed in U.S. Pat. Nos. 5,040,583, 5,005,625 and 5,156,921.
According to U.S. Pat. No. 5,040,583 the innerliner for a 15 inch tire with a single 55 mil thick halobutyl rubber innerliner ply weighs about 10 percent of the total weight of the tire. The patent discloses a laminated innerliner ply having one layer of a non-elastomeric barrier material of low permeability which reduces the 55 mil thick halobutyl innerliner ply to a 25 mil thick laminated innerliner ply. U.S. Pat. No. 5,156,921 discloses the use of a non-elastomeric barrier layer of polyvinylidene chloride or ethylenevinyl alcohol copolymer film to obtain equal or better air retention properties with a substantial reduction in thickness and mass over butyl or halobutyl rubber innerliner plies. In U.S. Pat. No. 5,005,625, a blend of an acrylonitrile/butadiene copolymer rubber with a butyl, chlorobutyl or bromobutyl rubber resulted in innerliner plies having a thickness in the range of about 31 mils to 78 mils. Therefore, it is known to vary the rubber compound in the innerliner ply to achieve more control on the inflation pressure loss. However, the most cost effective innerliner ply continues to be one made using a butyl or halobutyl compound rubber. The tire industry's preferred innerliner ply has a halobutyl compounded rubber at a constant thickness to achieve an overall acceptable of inflation pressure loss rate. The measure of acceptability is based on a "permeability coefficient" which will be described in more detail herein.
Tire endurance is reduced by loss of inflation pressure between intervals of pressure maintenance as well as deterioration of the rubber components with age. The deterioration of vulcanized rubber is well known to be accelerated in a high temperature environment.
If a constant thickness innerliner ply is used in a tire, the air will permeate more rapidly in those areas of the innerliner ply having a higher temperature. Permeability as a function of temperature will be discussed in more detail in the disclosure. Higher innerliner ply temperatures are found at locations relative to the higher internal tire temperature locations. Therefore, there is a need for an innerliner ply to have not only an average thickness (to reasonably maintain the tire's inflation pressure), but also an increased thickness in areas adjacent to high internal temperature locations within the tire (to limit the accelerated aging of the rubber components within the tire).
In the U.S. Pat. No. 3,495,645 the innerliner on one or both sides of the mid-sidewall region is made to have a greater thickness. This thickness increase leads to a reduction in vibrations transmitted to the wheel, which are generated by the tire's engagement with the ground. This innerliner feature further reduces the surge in driving torque transmitted through the tire. Thickness increases can be in either the shoulder region, the bead region, or both. These thickened areas are of a constant thickness twice to five times thicker than the mid-sidewall regions. Abrupt changes in thickness are relative to physical locations of the shoulder 15 and the bead region 6 regardless of the internal temperatures within the tire.
The art discloses inner tubes for tires have been used which have variations in their wall thickness. Two examples of this are in U.S. Pat. No. 2,588,097 and in United Kingdom Patent No. 180305 (GB). The greatest thickness of these inner tubes is at an outer tread portion and the smallest thickness is at a rim contact portion. These patents are directed to improved puncture resistance and do not teach or suggest the need for a profiled innerliner ply to achieve tire inflation pressure retention along with better aging protection.